1. Field of the Invention
The present invention is related to a method of manufacturing an analytical sample. In specific, the present invention is related to a method of manufacturing a SIMS sample.
2. Description of the Related Art
In production of industrial products, such as semiconductor devices, it is important to see concentration distribution of elements in the depth direction. For example, in a semiconductor element mounted over a semiconductor device, such as a field effect transistor (hereinafter referred to as a FET) or a thin film transistor (hereinafter referred to as a TFT), which is a kind of an FET; whether conditions of impurity introduction is suitable or not can be studied according to the concentration distribution of impurities in an impurity region in a semiconductor layer.
As a typical analysis method to reveal concentration distribution in the depth direction, secondary ion mass spectroscopy (hereinafter referred to as SIMS), auger electron spectroscopy (hereinafter referred to as AES), and X-ray photoelectron spectroscopy (hereinafter referred to as XPS) can be given. These analysis methods differ from one another in detection sensitivity, element discrimination capability, and the like; therefore, they are selected in accordance with the purpose. SIMS is an analysis method which is especially excellent in detection sensitivity and element discrimination capability.
In SIMS, a solid sample to be analyzed is irradiated with primary ions so that a surface of the solid sample is sputtered, and thus, ionized molecules or atoms are emitted from the solid sample surface and these ionized secondary ions are detected with a mass spectrometer. When a SIMS method is used, the concentration of impurity elements in a solid sample in which a thin film is formed on one main surface of a substrate and impurity elements are added to the thin film by ion implantation or the like, such as a TFT formed over an insulating substrate, can be analyzed. However, in analyzing such a solid sample, when the solid sample is irradiated with primary ions from a surface side of a thin film, the thin film is sputtered by primary ions, so that a crater is formed and the shape of the surface changes with time. Secondary ions are detected from side surfaces of a crater by a shape effect generated due to change in surface shape with time (crater edge effect); accordingly, accuracy of data in the depth direction decreases. Therefore, it is preferable to irradiate a rear surface (where a thin film is not formed) of a solid sample with primary ions instead of a front surface (where the thin film is formed) of the solid sample in order to precisely analyze the dose of impurity elements for imparting one conductivity in the thin film of the solid sample. To analyze a sample in such a manner, a following method may be employed: a surface of the top layer of an analytical sample is fixed to a table for polishing and a substrate is processed by polishing from a rear surface side by chemical mechanical polishing (hereinafter referred to as CMP) and the like to be as thin as about 1 μm, then the rear surface side of the substrate is irradiated with primary ions so as to be analyzed by SIMS.
SIMS analysis usually needs a structure which is not affected by charge build-up. Therefore, a material which is not electrically charged (e.g., silicon wafer) is necessarily laid under a film to be analyzed so that the film to be analyzed is not at a floating potential. Further, even in the case of using an insulating substrate, such as a glass substrate, if excessive charge build-up does not occur, the use of a neutralization gun provided for electrical neutralization (e.g., electron gun) allows a sample to be analyzed.
A polishing apparatus, such as a CMP apparatus is used for polishing the rear surface of the substrate. A CMP apparatus is provided with a polishing pad, a holding head (a head for fixing a sample), and a slurry (which contains a powder for mechanical polishing). As conditions for polishing, there are a load (force applied vertically to a polishing surface (a contact surface between the polishing pad and the holding head)), rotating speed, and the kind of the slurry, and these conditions are not easily determined. For example, it is difficult to polish the substrate to be as thin as about 1 μm while keeping the sample flat, which may need skills along with a wealth of experiences. In addition, the amount of polishing per unit time is reduced and polishing is conducted carefully, the time required is increased.
As described above, a method in which a rear surface of a substrate is polished and the polished surface is irradiated with primary ions for analysis by SIMS are troublesome for processing by polishing, and needs a great deal of time for pretreatment when many samples are evaluated. Further, advanced skill is needed for processing by polishing. Furthermore, flatness of the substrate may be lost due to the polishing step described above, which brings problems in precision in analysis by SIMS (e.g., see Patent Document 1: Japanese Published Patent Application No. H9-210885).